Clinical vision assessments

Diagnosis

There are many tests that may help ophthalmologists make a diagnosis of Retinitis Pigmentosa (RP). However, a diagnosis may require multiple tests to be carried out, potentially involving multiple visits to the eye clinic.

There are many forms of RP; determining which form an individual has is particularly difficult during the early stages of the condition.

Predicting the progression of RP is also difficult as the progression rate varies greatly amongst siblings with RP let alone RP patients in general [RNIB, 2013].

The various tests that may be used to diagnose RP have been described in this section (vision assessments) of the information.

A general eye examination

A general eye examination can be carried out at your local opticians by an optometrist. It can lasts an average of 20 to 30 minutes. There are three main aims of a general eye examination [RNIB, 2012]. These are:

Assessment of the outside of your eye (eye lids, eye movement, etc)

Assessment of the inside of your eye (retina)

To determine whether you need new glasses or contact lenses

A general eye examination involves:

Having your eye pressure measured

Being asked background questions also known as 'taking your history'

Having your visual acuity assessed

Eye pressure

Eye pressure is measured in most adults using an instrument called a 'non-contact tonometer'. Raised eye pressure can cause damage to the optic nerve [Specsavers, 2013]. Individuals who are found to have elevated eye pressure are referred to an ophthalmologist.

This assessment is carried out via the following steps:

The individual sits in a chair

The non-contact tonometer, which is placed on a table, is adjusted so that it is positioned in front of the individual

The individual then positions their head according to the chin rest and forehead strap, which supports the individual in keeping his/her head still during the assessment

The optometrist then blows a few puffs of air into the individual's eye. The instrument takes a measurement of the air which bounces back from the individual's eye; this represents the pressure inside the eye [Specsavers, 2013]

This is repeated for the second eye

Taking your history

In all cases, the individual's history is taken before the eye examination begins. The types of questions asked are as follows:

This is carried out to highlight any key points that the examiner should look out for or test further [RNIB, 2012]. Please do inform the examiner if you are wearing contact lenses.

Positioning

In general, most eye examinations require the individual to sit stationary in a chair whilst resting their chin on a chin rest and with their forehead resting against a support strap. Despite the positioning of the individual being similar in most cases, the actual machinery used for each test is different. The different machines are specialised for analysing different structures of the eye.

Eye drops

Examinations that assess structures inside the eye usually require eye drops to be administered into the individual's eyes; the eye drops dilate the pupils, enabling the examiner to assess more of the inner surface of the eye i.e. the retina [NHS Choices, 2013; Flanagan, 2009]. The eye drops take an average of 30 minutes to work.

If you are given eye drops, someone else will have to drive you home and you should also avoid excessive sunlight [Flanagan, 2009]. This is because the eye drops stop your eyes from focusing properly, resulting in your vision being slightly blurry for up to 12 hours. The effect of the eye drops usually wears off completely within 16-24 hours [NHS Choices, 2013; Flanagan, 2009].

If your family has a history of glaucoma then it is important that you tell the examiner before the eye drops are administered.

Always try and find out what assessments will be carried out during your appointment beforehand, so you can be prepared.

Best corrected visual acuity

Why the test is carried out

Visual acuity is a measure of best corrected distance vision [ASI, 2012]. An individual's visual acuity is assessed to determine the 'sharpness' of their central vision. There are various forms of this eye examination; however, the most common visual acuity assessment tool is the Snellen chart. The chart consists of random capital letters organised into 11 rows, decreasing in size when reading the chart from the top to the bottom.

How to prepare for the test

Avoid strain on the eye prior to the test.

How the assessment is carried out

The individual sits in a chair at a particular distance, usually 6 metres, from the chart

The individual is then asked to read from the top letter and to keep going until s/he can no longer read the letters in a particular line [ASI, 2012]

The examiner may then give the individual lenses of various strengths to see if vision improves or worsens. If it improves, you may need to wear glasses

In addition to an assessment with the Snellen chart, the ophthalmologist may quickly assess other parts of your eye by:

Shining a penlight on your pupil will show whether your pupil is able to respond to light properly [NIH]

Making you look in different directions whilst keeping your head still will assess your eye muscles [NIH]

Asking whether you can see specific objects at the side of your vision will assess your peripheral vision [NIH]

Understanding your results

Visual acuity is expressed as a fraction. The top number refers to the distance between yourself and the chart. This distance is usually six metres or 20 feet. The bottom number refers to the distance at which an individual with normal vision can read the last line you are able to read. If the top number is close to the bottom number i.e. 20/20, then the individual is said to have good visual acuity.

Slit-lamp biomicroscopy

Why the test is carried out

This form of microscopy enables the examiner to assess both the outer surface and inner surface of the eye [Lowther, G.E., 2010]. The outer surface structures, being assessed for any irregularities, are the cornea and the lens. When assessing the inner surface structure, the retina, the examiner also uses an additional lens. It is held between the slit-lamp and the patient's eye. Frequently, the examiner gives the patient eye drops to widen the pupil.

How to prepare for the test

Tell the examiner if you are currently on medication or whether you have an allergy to a particular medication. Also, if your family has a history of glaucoma then it is important that you tell the examiner prior to the eye drops being administered.

How the test is performed

This assessment is carried out via the following steps:

The individual sits in a chair

The ophthalmologist places eye drops in the individual's eye to dilate the pupil. The eye drops take an average of 30 minutes to work

The microscope, which is placed on a table, is adjusted so that it is positioned in front of the individual

The individual then positions their head according to the chin rest and forehead strap, supporting the individual in keeping his/her head still during the assessment

Using the slit-lamp microscope, as well as a handheld lens, held near the patient's eye, the examiner begins to assess the inside surface of the eye

In some cases, the examiner may want to assess the white surface of the eye and the tear layer. If so, the following steps are taken:

An eye drop containing a dye or a fine strip of paper stained with a dye may be applied to the white surface of the individual's eye [Lowther, G.E., 2010]

The individual is then asked to position themselves according to the chin and forehead rest attached to the microscope, as previously

The examiner then uses the slit lamp to analyse the eye surface further

The dye is washed out via blinking

Hardy-Rand Ritter colour plates

Why the test is carried out

The Hardy-Rand Ritter (HRR) colour plates is a test designed to detect, classify and estimate the degree of all three forms of colour blindness [Colbinder, 2006].

How the test is performed

The HRR colour plates are made up of coloured circles in the background in a range of sizes and lightness. Some plates are more difficult than others; this helps examine, if present, the severity of colour vision defect.

This assessment is carried out via the following steps:

The individual sits in a chair

The examiner then presents a plate to the individual

The individual then tells the examiner what they can see

This is repeated until all of the colour plates have been presented

Goldmann perimetry

Why the test is carried out

Goldmann perimetry is a test that assesses an individual's visual field sensitivity [Dersu et al, 2006]. A visual field test is an assessment ophthalmologists may use to diagnose RP. It is also an assessment the DVLA use to determine whether an individual, with an eye condition, is able to keep his/her license.

Since the results of the test is not analysed using a computerised system, the accuracy is dependent on both the individual and the examiner [Dersu et al, 2006]. It is important that the individual fully understands the test, maintains fixation on the point of focus and responds appropriately.

How to prepare for the test

Avoid strain on the eye prior to the test.

How the test is performed

This assessment is carried out via the following steps:

The individual sits in a chair with the apparatus placed directly in front of the individual

A patch is worn on the eye not being examined

The individual then positions their head according to the chin rest and forehead strap

The room lighting is then turned off

Once the individual is staring at a fixed spot, a moving stimulus (the stimulus chosen depends on the examiner - could be a flashing light) is presented to the individual in different regions of their visual field. Initially, the peripheral visual field is assessed, and then the central visual field is tested. This stimulus is controlled by the examiner

After each stimulus, the individual informs the examiner whether or not he/she saw the stimulus presented by pressing a buzzer or giving a verbal response

The examiner then records the individual's responses on paper in the form of a 'visual map'

This 'visual map' illustrates the individual's peripheral and central visual field sensitivity boundary. Assessment of each eye should not take longer than 10 minutes to avoid straining of the eye.

Retinal imaging

There are a few specialised cameras that are able to capture images of the interior surface of the eye.

Digital colour fundus photography

Why the test is carried out

Digital colour fundus photography is a form of eye examination that captures a coloured picture of the interior surface of the eye i.e. the retina. It is usually used to accurately information what is seen with the slit-lamp microscopy. This apparatus is made up of a specialised low power microscope with a specialised camera attached. It is used for the diagnosis of a condition (alongside retinal angiography (described below)), as well as for monitoring the progression of a condition [London Eye Hospital, 2011a].

How to prepare for the test

Avoid driving to your appointment as you will be given eye drops for your assessment; try and bring someone with you if you need to travel by vehicle. If you plan to go back to work, keep in mind that you will not be able to handle heavy machinery as you will not be able to focus your vision for up to 12 hours. If you have a family history of glaucoma, notify the examiner prior to the eye drops being administered.

If you are pregnant or have an allergy to a particular medicine, you must notify your ophthalmologist before having this assessment (see below).

Last but not least tell the examiner if you are wearing contact lenses.

How the test is performed

This assessment is carried out via the following steps:

The individual sits in a chair

The ophthalmologist places eye drops in the individual's eyes to dilate the pupils. The eye drops take an average of 30 minutes to work

The individual then positions their head according to the chin and forehead rest, which supports them in keeping their head still during the assessment

The apparatus is then positioned correctly in front of the individual, focusing on the target eye

Images can then begin to be captured. The camera works in the same fashion as a typical digital camera, however, the time taken to capture the images depends on how easily the images can be taken and how many images the clinician needs to take

Retinal angiography involves the use of the digital colour fundus camera following the same steps as above; however, a coloured dye (called fluorescein), is administered via an injection into the arm or hand of the individual prior to images being captured [Flanagan, 2009]. Once administered, a series of pictures are captured to see the flow of blood through the retina and choroid (structures at the back of the eye).

The dye induces a high contrast between the blood vessels and the retinal tissue, highlighting not only proper circulation in the retinal vessels but also any abnormalities such as the growth of new blood vessels in the macula (central region of the retina that is responsible for sharp detailed colour vision and does not have any blood vessels present), leakage from the retinal vessels or blockage.

The individual's urine will change colour, it will become more orange. This may last a day or two after the test and is nothing to be alarmed about, as the dye is leaving the individual's body.

Optical coherence tomography (OCT)

Why the test is carried out

The aim of the OCT is to capture micrometer resolution, three-dimensional images of both the front and back of the eye [London Eye Hospital, 2011b]. OCT enables the examiner to assess the different layers of the retina and to measure the overall thickness of the retina. This is important when determining or monitoring the extent of photoreceptor degeneration [London Eye Hospital, 2011b].

How the test is performed

This assessment is carried out via the following steps:

The individual sits in a chair

The individual then positions their head according to the chin and forehead rest, supporting them in keeping their head still during the assessment

Images can then begin to be captured

Fundus autofluorescence (FAF)

Why the test is carried out

FAF is a specialised camera that specifically monitors natural autofluorescence (absorbed light) in specific parts of the retina. High levels of autofluorescence may indicate degenerative change or injury, whilst low levels may indicate missing or non-functional retinal cells [Fleckenstein et al, 2010; Gerson, 2012]. This assessment is effective in confirming or eliminating age-related macular degeneration (AMD) as a diagnosis.

How to prepare for the test

Tell the examiner if you are currently on medication or whether you have an allergy to a particular medication. Also, if your family has a history of glaucoma then it is important that you tell the examiner prior to the eye drops being administered.

How the test is performed

This assessment is carried out via the following steps:

The individual sits in a chair

The individual then positions their head according to the chin and forehead rest, supporting them in keeping their head still during the assessment.

The examiner will then shine a blue light in the patient's eye

Images can then be captured and the flow or fluorescence is then measured

Electroretinogram (ERG)

Why the test is carried out

In response to light, the retina sends messages to the brain in an attempt to describe what can be seen in the individual's visual field. These messages are sent in the form electrical activity along neurons that run from the retina to the part of the brain that is responsible for visual information, called the visual cortex. It is this electrical activity of the retina that an ERG measures [Wolpert and Tsang, 2011]. This assessment can detect disorders of the retina and is a critical diagnostic assessment for RP.

How the test is performed

This assessment is carried out via the following steps:

Eye drops are administered to prevent the individual feeling any discomfort

The individual's eye is held open with a device called a retractor; this will make sure the examination is completed successfully

Electrodes are placed on the cornea and the skin near the eye

The individual's eyes are exposed to a visual stimulus (light source)

The electrodes record the electrical activity triggered by the stimuli, which is displayed in a graph format

This assessment is carried out, initially in a normally lit room and then the individual is taken into a dark room, given 20 minutes to adjust, and then the examination is carried out again following the same steps.

In total, this assessment takes up to an hour to complete.

This page is not a substitute for a consultation with a health professional and should not to be used as a means of diagnosing a condition.